A global catastrophic risk is a hypothetical future event which could damage human well-being on a global scale,[2] even crippling or destroying modern civilization.[3] An event that could cause human extinction or permanently and drastically curtail humanity's potential is known as an existential risk.[4]

A "global catastrophic risk" is any risk that is at least "global" in scope, and is not subjectively "imperceptible" in intensity. Those that are at least "trans-generational" (affecting all future generations) in scope and "terminal"[clarification needed] in intensity are classified as existential risks. While a global catastrophic risk may kill the vast majority of life on earth, humanity could still potentially recover. An existential risk, on the other hand, is one that either destroys humanity (and, presumably, all but the most rudimentary species of non-human lifeforms and/or plant life) entirely or at least prevents any chance of civilization recovering.[6]

Researchers experience difficulty in studying near human extinction directly, since humanity has never been destroyed before.[8] While this does not mean that it will not be in the future, it does make modelling existential risks difficult, due in part to survivorship bias. However, civilizations vanished rather frequently in human history.

Some risks are due to phenomena that have occurred in earth's past and left a geological record. Together with contemporary observations, it is possible to make informed estimates of the likelihood such events will occur in the future. For example, an extinction-level comet or asteroid impact event before the year 2100 has been estimated at one-in-a-million.[9][10][further explanation needed]Supervolcanoes are another example. There are several known supervolcanos, including Mt. Toba, which some say almost wiped out humanity at the time of its last eruption. The geologic record suggests this particular supervolcano re-erupts about every 50,000 years.[11][12][further explanation needed]

Without the benefit of geological records and direct observation, the relative danger posed by other threats is much more difficult to calculate. In addition, it is one thing to estimate the likelihood of an event taking place, something else to assess how likely an event will cause extinction if it does occur, and most difficult of all, the risk posted by synergistic effects of multiple events taking place simultaneously.[citation needed]

Given the limitations of ordinary calculation and modeling, expert elicitation is frequently used instead to obtain probability estimates.[13] In 2008, an informal survey of experts on different global catastrophic risks at the Global Catastrophic Risk Conference at the University of Oxford suggested a 19% chance of human extinction by the year 2100. The conference report cautions that the results should be taken "with a grain of salt", the results were not meant to capture all large risks and did not include things like climate change, and the results likely reflect many cognitive biases of the conference participants.[14]

The 2016 annual report by the Global Challenges Foundation estimates that an average American is more than five times more likely to die during a human-extinction event than in a car crash.[16][17]

There are significant methodological challenges in estimating these risks with precision. Most attention has been given to risks to human civilization over the next 100 years, but forecasting for this length of time is difficult. The types of threats posed by nature have been argued to be relatively constant, though this has been disputed,[18] and new risks could be discovered. Anthropogenic threats, however, are likely to change dramatically with the development of new technology; while volcanoes have been a threat throughout history, nuclear weapons have only been an issue since the 20th century. Historically, the ability of experts to predict the future over these timescales has proved very limited. Man-made threats such as nuclear war or nanotechnology are harder to predict than natural threats, due to the inherent methodological difficulties in the social sciences. In general, it is hard to estimate the magnitude of the risk from this or other dangers, especially as both international relations and technology can change rapidly.

Existential risks pose unique challenges to prediction, even more than other long-term events, because of observation selection effects. Unlike with most events, the failure of a complete extinction event to occur in the past is not evidence against their likelihood in the future, because every world that has experienced such an extinction event has no observers, so regardless of their frequency, no civilization observes existential risks in its history.[8] These anthropic issues can be avoided by looking at evidence that does not have such selection effects, such as asteroid impact craters on the Moon, or directly evaluating the likely impact of new technology.[5]

In addition to known and tangible risks, unforeseeable black swan extinction events may occur, presenting an additional methodological problem.[19]

Some scholars have strongly favored reducing existential risk on the grounds that it greatly benefits future generations. Derek Parfit argues that extinction would be a great loss because our descendants could potentially survive for four billion years before the expansion of the Sun makes the Earth uninhabitable.[20][21]Nick Bostrom argues that there is even greater potential in colonizing space. If future humans colonize space, they may be able to support a very large number of people on other planets, potentially lasting for trillions of years.[6] Therefore, reducing existential risk by even a small amount would have a very significant impact on the expected number of people who will exist in the future.

Exponential discounting might make these future benefits much less significant. However, Jason Matheny has argued that such discounting is inappropriate when assessing the value of existential risk reduction.[9]

Some economists have discussed the importance of global catastrophic risks, though not existential risks. Martin Weitzman argues that most of the expected economic damage from climate change may come from the small chance that warming greatly exceeds the mid-range expectations, resulting in catastrophic damage.[22]Richard Posner has argued that we are doing far too little, in general, about small, hard-to-estimate risks of large-scale catastrophes.[23]

Scope insensitivity influences how bad people consider the extinction of the human race to be. For example, when people are motivated to donate money to altruistic causes, the quantity they are willing to give does not increase linearly with the magnitude of the issue: people are roughly as concerned about 200,000 birds getting stuck in oil as they are about 2,000.[25] Similarly, people are often more concerned about threats to individuals than to larger groups.[24]

There are economic reasons that can explain why so little effort is going into existential risk reduction. It is a global good, so even if a large nation decreases it, that nation will only enjoy a small fraction of the benefit of doing so. Furthermore, the vast majority of the benefits may be enjoyed by far future generations, and though these quadrillions of future people would in theory perhaps be willing to pay massive sums for existential risk reduction, no mechanism for such a transaction exists.[5]

Some sources of catastrophic risk are natural, such as meteor impacts or supervolcanoes. Some of these have caused mass extinctions in the past. On the other hand, some risks are man-made, such as global warming,[26] environmental degradation, engineered pandemics and nuclear war.[27]

The Cambridge Project at Cambridge University states that the "greatest threats" to the human species are man-made; they are artificial intelligence, global warming, nuclear war, and rogue biotechnology.[28] The Future of Humanity Institute also states that human extinction is more likely to result from anthropogenic causes than natural causes.[5][29]

It has been suggested that learning computers that rapidly become superintelligent may take unforeseen actions, or that robots would out-compete humanity (one technological singularity scenario).[30] Because of its exceptional scheduling and organizational capability and the range of novel technologies it could develop, it is possible that the first Earth superintelligence to emerge could rapidly become matchless and unrivaled: conceivably it would be able to bring about almost any possible outcome, and be able to foil virtually any attempt that threatened to prevent it achieving its objectives.[31] It could eliminate, wiping out if it chose, any other challenging rival intellects; alternatively it might manipulate or persuade them to change their behavior towards its own interests, or it may merely obstruct their attempts at interference.[31] In Bostrom's book, Superintelligence: Paths, Dangers, Strategies, he defines this as the control problem.[32] Physicist Stephen Hawking, Microsoft founder Bill Gates and SpaceX founder Elon Musk have echoed these concerns, with Hawking theorizing that this A.I. could "spell the end of the human race".[33]

In 2009, the Association for the Advancement of Artificial Intelligence (AAAI) hosted a conference to discuss whether computers and robots might be able to acquire any sort of autonomy, and how much these abilities might pose a threat or hazard. They noted that some robots have acquired various forms of semi-autonomy, including being able to find power sources on their own and being able to independently choose targets to attack with weapons. They also noted that some computer viruses can evade elimination and have achieved "cockroach intelligence." They noted that self-awareness as depicted in science-fiction is probably unlikely, but that there were other potential hazards and pitfalls.[34] Various media sources and scientific groups have noted separate trends in differing areas which might together result in greater robotic functionalities and autonomy, and which pose some inherent concerns.[35][36]

A survey of AI experts estimated that the chance of human-level machine learning having an "extremely bad (e.g., human extinction)" long-term effect on humanity is 5%.[37] A 2008 survey by the Future of Humanity Institute estimated a 5% probability of extinction by superintelligence by 2100.[14]Eliezer Yudkowsky believes that risks from artificial intelligence are harder to predict than any other known risks due to bias from anthropomorphism. Since people base their judgments of artificial intelligence on their own experience, he claims that they underestimate the potential power of AI.[38]

A biotechnology catastrophe may be caused by accidentally releasing a genetically engineered organism from controlled environments, by the planned release of such an organism which then turns out to have unforeseen and catastrophic interactions with essential natural or agro-ecosystems, or by intentional usage of biological agents in biological warfare or bioterrorism attacks.[39] Pathogens may be intentionally or unintentionally genetically modified to change virulence and other characteristics.[39] For example, a group of Australian researchers unintentionally changed characteristics of the mousepox virus while trying to develop a virus to sterilize rodents.[39] The modified virus became highly lethal even in vaccinated and naturally resistant mice.[40][41] The technological means to genetically modify virus characteristics are likely to become more widely available in the future if not properly regulated.[39]

Terrorist applications of biotechnology have historically been infrequent. To what extent this is due to a lack of capabilities or motivation is not resolved.[39] However, given current development, more risk from novel, engineered pathogens is to be expected in the future.[39] Exponential growth has been observed in the biotechnology sector, and Noun and Chyba predict that this will lead to major increases in biotechnological capabilities in the coming decades.[39] They argue that risks from biological warfare and bioterrorism are distinct from nuclear and chemical threats because biological pathogens are easier to mass-produce and their production is hard to control (especially as the technological capabilities are becoming available even to individual users).[39] In 2008, a survey by the Future of Humanity Institute estimated a 2% probability of extinction from engineered pandemics by 2100.[14]

Cyberattacks have the potential to destroy everything from personal data to electric grids. Christine Peterson, co-founder and past president of the Foresight Institute, believes a cyberattack on electric grids has the potential to be a catastrophic risk.[42]

An October 2017 report published in The Lancet stated that toxic air, water, soils, and workplaces were collectively responsible for 9 million deaths worldwide in 2015, particularly from air pollution which was linked to deaths by increasing susceptibility to non-infectious diseases, such as heart disease, stroke, and lung cancer.[47] The report warned that the pollution crisis was exceeding "the envelope on the amount of pollution the Earth can carry" and “threatens the continuing survival of human societies”.[47]

Global warming refers to the warming caused by human technology since the 19th century or earlier. Projections of future climate change suggest further global warming, sea level rise, and an increase in the frequency and severity of some extreme weather events and weather-related disasters. Effects of global warming include loss of biodiversity, stresses to existing food-producing systems, increased spread of known infectious diseases such as malaria, and rapid mutation of microorganisms. In November 2017, a statement by 15,364 scientists from 184 countries indicated that increasing levels of greenhouse gases from use of fossil fuels, human population growth, deforestation, and overuse of land for agricultural production, particularly by farming ruminants for meat consumption, are trending in ways that forecast an increase in human misery over coming decades.[3]

Ever since Georgescu-Roegen and Daly published these views, various scholars in the field have been discussing the existential impossibility of allocating earth's finite stock of mineral resources evenly among an unknown number of present and future generations. This number of generations is likely to remain unknown to us, as there is no way — or only little way — of knowing in advance if or when mankind will ultimately face extinction. In effect, any conceivable intertemporal allocation of the stock will inevitably end up with universal economic decline at some future point.[59]:253–256[60]:165[61]:168–171[62]:150–153[63]:106–109[64]:546–549[65]:142–145[66]

Many nanoscale technologies are in development or currently in use.[67] The only one that appears to pose a significant global catastrophic risk is molecular manufacturing, a technique that would make it possible to build complex structures at atomic precision.[68] Molecular manufacturing requires significant advances in nanotechnology, but once achieved could produce highly advanced products at low costs and in large quantities in nanofactories of desktop proportions.[67][68] When nanofactories gain the ability to produce other nanofactories, production may only be limited by relatively abundant factors such as input materials, energy and software.[67]

Molecular manufacturing could be used to cheaply produce, among many other products, highly advanced, durable weapons.[67] Being equipped with compact computers and motors these could be increasingly autonomous and have a large range of capabilities.[67]

Chris Phoenix and Treder classify catastrophic risks posed by nanotechnology into three categories:

From augmenting the development of other technologies such as AI and biotechnology.

By enabling mass-production of potentially dangerous products that cause risk dynamics (such as arms races) depending on how they are used.

Several researchers state that the bulk of risk from nanotechnology comes from the potential to lead to war, arms races and destructive global government.[40][67][69] Several reasons have been suggested why the availability of nanotech weaponry may with significant likelihood lead to unstable arms races (compared to e.g. nuclear arms races):

A large number of players may be tempted to enter the race since the threshold for doing so is low;[67]

The ability to make weapons with molecular manufacturing will be cheap and easy to hide;[67]

Therefore, lack of insight into the other parties' capabilities can tempt players to arm out of caution or to launch preemptive strikes;[67][70]

Molecular manufacturing may reduce dependency on international trade,[67] a potential peace-promoting factor;

Wars of aggression may pose a smaller economic threat to the aggressor since manufacturing is cheap and humans may not be needed on the battlefield.[67]

Since self-regulation by all state and non-state actors seems hard to achieve,[71] measures to mitigate war-related risks have mainly been proposed in the area of international cooperation.[67][72] International infrastructure may be expanded giving more sovereignty to the international level. This could help coordinate efforts for arms control. International institutions dedicated specifically to nanotechnology (perhaps analogously to the International Atomic Energy Agency IAEA) or general arms control may also be designed.[72] One may also jointly make differential technological progress on defensive technologies, a policy that players should usually favour.[67] The Center for Responsible Nanotechnology also suggests some technical restrictions.[73] Improved transparency regarding technological capabilities may be another important facilitator for arms-control.

Grey goo is another catastrophic scenario, which was proposed by Eric Drexler in his 1986 book Engines of Creation[74] and has been a theme in mainstream media and fiction.[75][76] This scenario involves tiny self-replicating robots that consume the entire biosphere using it as a source of energy and building blocks. Nowadays, however, nanotech experts—including Drexler—discredit the scenario. According to Phoenix, a "so-called grey goo could only be the product of a deliberate and difficult engineering process, not an accident".[77]

The scenarios that have been explored most frequently are nuclear warfare and doomsday devices. Although the probability of a nuclear war per year is slim, Professor Martin Hellman has described it as inevitable in the long run; unless the probability approaches zero, inevitably there will come a day when civilization's luck runs out.[78] During the Cuban missile crisis, U.S. president John F. Kennedy estimated the odds of nuclear war at "somewhere between one out of three and even".[79] The United States and Russia have a combined arsenal of 14,700 nuclear weapons,[80] and there is an estimated total of 15,700 nuclear weapons in existence worldwide.[80] Beyond nuclear, other military threats to humanity include biological warfare (BW). By contrast, chemical warfare, while able to create multiple local catastrophes, is unlikely to create a global one.

Nuclear war could yield unprecedented human death tolls and habitat destruction. Detonating large numbers of nuclear weapons would have an immediate, short term and long-term effects on the climate, causing cold weather and reduced sunlight and photosynthesis[81] that may generate significant upheaval in advanced civilizations.[82] However, while popular perception sometimes takes nuclear war as "the end of the world", experts assign low probability to human extinction from nuclear war.[83][84] In 1982, Brian Martin estimated that a US–Soviet nuclear exchange might kill 400–450 million directly, mostly in the United States, Europe and Russia, and maybe several hundred million more through follow-up consequences in those same areas.[83] In 2008, a survey by the Future of Humanity Institute estimated a 4% probability of extinction from warfare by 2100, with a 1% chance of extinction from nuclear warfare.[14]

The 20th century saw a rapid increase in human population due to medical developments and massive increases in agricultural productivity[85] such as the Green Revolution.[86] Between 1950 and 1984, as the Green Revolution transformed agriculture around the globe, world grain production increased by 250%. The Green Revolution in agriculture helped food production to keep pace with worldwide population growth or actually enabled population growth. The energy for the Green Revolution was provided by fossil fuels in the form of fertilizers (natural gas), pesticides (oil), and hydrocarbon-fueled irrigation.[87] David Pimentel, professor of ecology and agriculture at Cornell University, and Mario Giampietro, senior researcher at the National Research Institute on Food and Nutrition (INRAN), place in their 1994 study Food, Land, Population and the U.S. Economy the maximum U.S. population for a sustainable economy at 200 million. To achieve a sustainable economy and avert disaster, the United States must reduce its population by at least one-third, and world population will have to be reduced by two-thirds, says the study.[88]

The authors of this study believe that the mentioned agricultural crisis will begin to have an effect on the world after 2020, and will become critical after 2050. Geologist Dale Allen Pfeiffer claims that coming decades could see spiraling food prices without relief and massive starvation on a global level such as never experienced before.[89][90]

Wheat is humanity's third-most-produced cereal. Extant fungal infections such as Ug99[91] (a kind of stem rust) can cause 100% crop losses in most modern varieties. Little or no treatment is possible and infection spreads on the wind. Should the world's large grain-producing areas become infected, the ensuing crisis in wheat availability would lead to price spikes and shortages in other food products.[92]

Several asteroids have collided with earth in recent geological history. The Chicxulub asteroid, for example, was around 6 miles in diameter and is theorized to have caused the extinction of the non-avian dinosaurs 66 million years ago at the end of the Cretaceous. No sufficiently large asteroid currently exists in an Earth-crossing orbit; however, a comet of sufficient size to cause human extinction could impact the Earth, though the annual probability may be less than 10−8.[93] Geoscientist Brian Toon estimates that while a few people, such as "some fishermen in Costa Rica", could plausibly survive a 6-mile meteorite, a 60-mile meteorite would be large enough to "incinerate everybody".[94] Asteroids with around a 1 km diameter have impacted the Earth on average once every 500,000 years; these are probably too small to pose an extinction risk, but might kill billions of people.[93][95] Larger asteroids are less common. Small near-Earth asteroids are regularly observed and can impact anywhere on the Earth injuring local populations.[96] As of 2013, Spaceguard estimates it has identified 95% of all NEOs over 1 km in size.[97]

A number of astronomical threats have been identified. Massive objects, e.g. a star, large planet or black hole, could be catastrophic if a close encounter occurred in the Solar System. In April 2008, it was announced that two simulations of long-term planetary movement, one at the Paris Observatory and the other at the University of California, Santa Cruz, indicate a 1% chance that Mercury's orbit could be made unstable by Jupiter's gravitational pull sometime during the lifespan of the Sun. Were this to happen, the simulations suggest a collision with Earth could be one of four possible outcomes (the others being Mercury colliding with the Sun, colliding with Venus, or being ejected from the Solar System altogether). If Mercury were to collide with Earth, all life on Earth could be obliterated entirely: an asteroid 15 km wide is believed to have caused the extinction of the non-avian dinosaurs, whereas Mercury is 4,879 km in diameter.[109]

Another cosmic threat is a gamma-ray burst, typically produced by a supernova when a star collapses inward on itself and then "bounces" outward in a massive explosion. Under certain circumstances, these events are thought to produce massive bursts of gamma radiation emanating outward from the axis of rotation of the star. If such an event were to occur oriented towards the Earth, the massive amounts of gamma radiation could significantly affect the Earth's atmosphere and pose an existential threat to all life. Such a gamma-ray burst may have been the cause of the Ordovician–Silurian extinction events. Neither this scenario nor the destabilization of Mercury's orbit are likely in the foreseeable future.[110]

A powerful solar flare or solar superstorm, which is a drastic and unusual decrease or increase in the Sun's power output, could have severe consequences for life on Earth.[111][112]

If our universe lies within a false vacuum, a bubble of lower-energy vacuum could come to exist by chance or otherwise in our universe, and catalyze the conversion of our universe to a lower energy state in a volume expanding at nearly the speed of light, destroying all that we know without forewarning. Such an occurrence is called vacuum decay.[113][114]

The most predictable outcome for the future of the Earth is the Sun's expansion into a red giant star. The Sun will be about 12 billion years old and expand to swallow both Mercury and Venus, reaching a maximum radius of 1.2 AU (180,000,000 km). The Earth will interact tidally with the Sun's outer atmosphere, which would serve to decrease Earth's orbital radius. Drag from the chromosphere of the Sun would also reduce the Earth's orbit. These effects will act to counterbalance the effect of mass loss by the Sun, and the Earth will probably be engulfed by the Sun.[115]

Intelligent extraterrestrial life, if existent, could invade Earth[116] either to exterminate and supplant human life, enslave it under a colonial system, steal the planet's resources, or destroy the planet altogether.

An article in The New York Times discussed the possible threats for humanity of intentionally sending messages aimed at extraterrestrial life into the cosmos in the context of the SETI efforts. Several renowned public figures such as Stephen Hawking and Elon Musk have argued against sending such messages on the grounds that extraterrestrial civilizations with technology are probably far more advanced than humanity and could pose an existential threat to humanity.[119]

Numerous historical examples of pandemics[120] had a devastating effect on a large number of people. The present, unprecedented scale and speed of human movement make it more difficult than ever to contain an epidemic through local quarantines, and other sources of uncertainty and the evolving nature of the risk means natural pandemics may pose a realistic threat to human civilization.[18]

There are several classes of argument about the likelihood of pandemics. One class of argument about likelihood stems from the history of pandemics, where the limited size of historical pandemics is evidence that larger pandemics are unlikely. This argument has been disputed on several grounds, including the changing risk due to changing population and behavioral patterns among humans, the limited historical record, and the existence of an anthropic bias.[18]

Another argument about the likelihood of pandemics is based on an evolutionary model that predicts that naturally evolving pathogens will ultimately develop an upper limit to their virulence.[121] This is because pathogens with high enough virulence quickly kill their hosts and reduce their chances of spread the infection to new hosts or carriers.[122] This model has limits, however, because the fitness advantage of limited virulence is primarily a function of a limited number of hosts. Any pathogen with a high virulence, high transmission rate and long incubation time may have already caused a catastrophic pandemic before ultimately virulence is limited through natural selection. Additionally, a pathogen that infects humans as a secondary host and primarily infects another species (a zoonosis) has no constraints on its virulence in people, since the accidental secondary infections do not affect its evolution.[123] Lastly, in models where virulence level and rate of transmission are related, high levels of virulence can evolve.[124] Virulence is instead limited by the existence of complex populations of hosts with different susceptibilities to infection, or by some hosts being geographically isolated.[121] The size of the host population and competition between different strains of pathogens can also alter virulence.[125]

Neither of these arguments is applicable to bioengineered pathogens, and this poses entirely different risks of pandemics. Experts have concluded that "Developments in science and technology could significantly ease the development and use of high consequence biological weapons," and these "highly virulent and highly transmissible [bio-engineered pathogens] represent new potential pandemic threats."[126]

Climate change refers to a lasting change in the Earth's climate. The climate has ranged from ice ages to warmer periods when palm trees grew in Antarctica. It has been hypothesized that there was also a period called "snowball Earth" when all the oceans were covered in a layer of ice. These global climatic changes occurred slowly, prior to the rise of human civilization about 10 thousand years ago near the end of the last Major Ice Age when the climate became more stable. However, abrupt climate change on the decade time scale has occurred regionally. Since civilization originated during a period of stable climate, a natural variation into a new climate regime (colder or hotter) could pose a threat to civilization.[127][128]

In the history of the Earth, many ice ages are known to have occurred. An ice age would have a serious impact on civilization because vast areas of land (mainly in North America, Europe, and Asia) could become uninhabitable. Currently, the world is in an interglacial period within a much older glacial event. The last glacial expansion ended about 10,000 years ago, and all civilizations evolved later than this. Scientists do not predict that a natural ice age will occur anytime soon.[citation needed] The amount of heat trapping gases emitted into Earth's Oceans and atmosphere will prevent the next ice age, which otherwise would begin in around 50,000 years, and likely more glacial cycles.[129][130]

A geological event such as massive flood basalt, volcanism, or the eruption of a supervolcano[131] could lead to a so-called volcanic winter, similar to a nuclear winter. One such event, the Toba eruption,[132] occurred in Indonesia about 71,500 years ago. According to the Toba catastrophe theory,[133] the event may have reduced human populations to only a few tens of thousands of individuals. Yellowstone Caldera is another such supervolcano, having undergone 142 or more caldera-forming eruptions in the past 17 million years.[134]
A massive volcano eruption would eject extraordinary volumes of volcanic dust, toxic and greenhouse gases into the atmosphere with serious effects on global climate (towards extreme global cooling: volcanic winter if short-term, and ice age if long-term) or global warming (if greenhouse gases were to prevail).

When the supervolcano at Yellowstone last erupted 640,000 years ago, the thinnest layers of the ash ejected from the caldera spread over most of the United States west of the Mississippi River and part of northeastern Mexico. The magma covered much of what is now Yellowstone National Park and extended beyond, covering much of the ground from Yellowstone River in the east to the Idaho falls in the west, with some of the flows extending north beyond Mammoth Springs.[135]

According to a recent study, if the Yellowstone caldera erupted again as a supervolcano, an ash layer one to three millimeters thick could be deposited as far away as New York, enough to "reduce traction on roads and runways, short out electrical transformers and cause respiratory problems". There would be centimeters of thickness over much of the U.S. Midwest, enough to disrupt crops and livestock, especially if it happened at a critical time in the growing season. The worst-affected city would likely be Billings, Montana, population 109,000, which the model predicted would be covered with ash estimated as 1.03 to 1.8 meters thick.[136]

The main long-term effect is through global climate change, which reduces the temperature globally by about 5–15 degrees C for a decade, together with the direct effects of the deposits of ash on their crops. A large supervolcano like Toba would deposit one or two meters thickness of ash over an area of several million square kilometers.(1000 cubic kilometers is equivalent to a one-meter thickness of ash spread over a million square kilometers). If that happened in some densely populated agricultural area, such as India, it could destroy one or two seasons of crops for two billion people.[137]

However, Yellowstone shows no signs of a supereruption at present, and it is not certain that a future supereruption will occur there.[138][139]

Research published in 2011 finds evidence that massive volcanic eruptions caused massive coal combustion, supporting models for significant generation of greenhouse gases. Researchers have suggested that massive volcanic eruptions through coal beds in Siberia would generate significant greenhouse gases and cause a runaway greenhouse effect.[140] Massive eruptions can also throw enough pyroclastic debris and other material into the atmosphere to partially block out the sun and cause a volcanic winter, as happened on a smaller scale in 1816 following the eruption of Mount Tambora, the so-called Year Without a Summer. Such an eruption might cause the immediate deaths of millions of people several hundred miles from the eruption, and perhaps billions of death worldwide, due to the failure of the monsoons,[141] resulting in major crop failures causing starvation on a profound scale.[141]

A much more speculative concept is the verneshot: a hypothetical volcanic eruption caused by the buildup of gas deep underneath a craton. Such an event may be forceful enough to launch an extreme amount of material from the crust and mantle into a sub-orbital trajectory.

The Svalbard Global Seed Vault is buried 400 feet (120 m) inside a mountain on an island in the Arctic. It is designed to hold 2.5 billion seeds from more than 100 countries as a precaution to preserve the world's crops. The surrounding rock is −6 °C (21 °F) (as of 2015) but the vault is kept at −18 °C (0 °F) by refrigerators powered by locally sourced coal.[144][145]

More speculatively, if society continues to function and if the biosphere remains habitable, calorie needs for the present human population might in theory be met during an extended absence of sunlight, given sufficient advance planning. Conjectured solutions include growing mushrooms on the dead plant biomass left in the wake of the catastrophe, converting cellulose to sugar, or feeding natural gas to methane-digesting bacteria.[146][147]

Insufficient global governance creates risks in the social and political domain, but the governance mechanisms develop more slowly than technological and social change. There are concerns from governments, the private sector, as well as the general public about the lack of governance mechanisms to efficiently deal with risks, negotiate and adjudicate between diverse and conflicting interests. This is further underlined by an understanding of the interconnectedness of global systemic risks.[148] In absence or anticipation of global governance, national governments can act individually to better understand, mitigate and prepare for global catastrophes.[149]

In 2018, the Club of Rome submitted a plan to the European Parliament, urging to address the existential threat from climate change more forcefully, calling for a collaborative climate action afford.[150]

The Bulletin of the Atomic Scientists (est. 1945) is one of the oldest global risk organizations, founded after the public became alarmed by the potential of atomic warfare in the aftermath of WWII. It studies risks associated with nuclear war and energy and famously maintains the Doomsday Clock established in 1947. The Foresight Institute (est. 1986) examines the risks of nanotechnology and its benefits. It was one of the earliest organizations to study the unintended consequences of otherwise harmless technology gone haywire at a global scale. It was founded by K. Eric Drexler who postulated "grey goo".[151][152]

Beginning after 2000, a growing number of scientists, philosophers and tech billionaires created organizations devoted to studying global risks both inside and outside of academia.[153]

University-based organizations include the Future of Humanity Institute (est. 2005) which researches the questions of humanity's long-term future, particularly existential risk. It was founded by Nick Bostrom and is based at Oxford University. The Centre for the Study of Existential Risk (est. 2012) is a Cambridge-based organization which studies four major technological risks: artificial intelligence, biotechnology, global warming and warfare. All are man-made risks, as Huw Price explained to the AFP news agency, "It seems a reasonable prediction that some time in this or the next century intelligence will escape from the constraints of biology". He added that when this happens "we're no longer the smartest things around," and will risk being at the mercy of "machines that are not malicious, but machines whose interests don't include us."[162]Stephen Hawking was an acting adviser. The Millennium Alliance for Humanity and the Biosphere is a Stanford University-based organization focusing on many issues related to global catastrophe by bringing together members of academic in the humanities.[163][164] It was founded by Paul Ehrlich among others.[165] Stanford University also has the Center for International Security and Cooperation focusing on political cooperation to reduce global catastrophic risk.[166] The Center for Security and Emerging Technology was established in January 2019 at Georgetown's Walsh School of Foreign Service and will focus on policy research of emerging technologies with an initial emphasis on artificial intelligence.[167] They received a grant of 55M USD from Good Ventures as suggested by the Open Philanthropy Project.[167]

Other risk assessment groups are based in or are part of governmental organizations. The World Health Organization (WHO) includes a division called the Global Alert and Response (GAR) which monitors and responds to global epidemic crisis.[168] GAR helps member states with training and coordination of response to epidemics.[169] The United States Agency for International Development (USAID) has its Emerging Pandemic Threats Program which aims to prevent and contain naturally generated pandemics at their source.[170] The Lawrence Livermore National Laboratory has a division called the Global Security Principal Directorate which researches on behalf of the government issues such as bio-security and counter-terrorism.[171]

^"Steam Explosions, Earthquakes, and Volcanic Eruptions—What's in Yellowstone's Future?". USGS Yellowstone Volcanic Observatory. The USGS puts it like this: "If another large caldera-forming eruption were to occur at Yellowstone, its effects would be worldwide. Thick ash deposits would bury vast areas of the United States, and injection of huge volumes of volcanic gases into the atmosphere could drastically affect global climate. Fortunately, the Yellowstone volcanic system shows no signs that it is headed toward such an eruption. The probability of a large caldera-forming eruption within the next few thousand years is exceedingly low."